Method and construction of filters and pre-filters for extending the life cycle of the filter bodies therein

ABSTRACT

A method of utilizing a filter having a filter body removing solid particles from a fluid, in order to extend the life cycle of the filter body before cleaning or replacement thereof is required, characterized in applying to the fluid, upstream of the filter body, an AC electrical field of a frequency and intensity to agglomerate solid particles in the fluid reaching the filter body, such as to reduce the number of smaller particles in the fluid tending to clog the filter body. Also described are a plurality of filter constructions for extending the life cycle of the filter body in accordance with the above method.

RELATED PATENT APPLICATION

This application is a National Phase Application of PCT/IL2004/000107having International Filing Date of 4 Feb. 2004, which claims thebenefit of U.S. Provisional Patent Application No. 60/446,440 filed 12Feb. 2003. The contents of the above Application are all incorporatedherein by reference.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to filters, and particularly to a method,filter and pre-filter for extending the life cycle of the filter bodybefore cleaning or replacement is required. The invention isparticularly useful with respect to filters of fibrous material, such aspaper, for removing solid particles from air or other gases, and theinvention is therefore described below with respect to suchapplications.

The term “air filter” as used herein is intended to include filters notonly for air, but also for other gases; the term “paper filter” as usedherein is intended to include not only folded paper filters but alsoother kinds of fiber filters; and the term “solid particles” as usedherein is intended to include not only dust, but also other micro-bodiessuch as aerosols.

One of the largest applications for air filters at the present time isfor cleaning the air supplied to internal combustion engines and aircompressors. Another popular application is for clean rooms wherein HighEfficiency Particle Arrestance (HEPA) filters are mostly used. The mostcommon type of air filters is the paper filter, made up of multiplelayers of folded paper or other fiber.

A method for cleaning paper filters by air jets directed through thefilter in a direction opposed to the normal airflow path was disclosedin Copley U.S. Pat. No. 4,364,751 and Gillingham et al. U.S. Pat. No.4,504,293. Both used a pulse jetting formed by a system of valves andnozzles. Since dust particles are retained by the fiber mesh quitetightly, the pulse jet cleaning method can not provide sufficientrecovery of the filter, and after only a limited number of cleaningcycles at most, the filter must be discarded and replaced with a newone.

U.S. Pat. Nos. 5,797,978 and 6,391,097 disclose a novel air filtrationtechnology based on plastic discs secured in a tight stack. The discsfaces are made with a pattern forming small filtering passageways. Anelectrode metallic layer is embedded in the plastics and is connected toa DC voltage source in order to generate an electrostatic field betweenthe discs. This field acts on the dust particles to divert them from theair stream and to settle them on the faces of the plastic discs. Such adisc structure can sustain pressure load higher than the fiber materialand accordingly enables easy cleaning by reverse air impulses. A novelfilter system that includes an air impulse generator for cleaning thefilter many times is described in U.S. Pat. No. 6,641,646.

However, paper filters are generally disposable; that is when cloggedthey generally must be discarded and replaced with new ones. Cleaningpaper filters by airflow in the opposite direction is generally not soeffective when done by air jets and might cause rupture of the filter ifexposed to a blast of air impulse. Periodical replacement of disposableair filters is costly and constitutes an ecological burden.

OBJECTS AND BRIEF SUMMARY OF THE PRESENT INVENTION

An object of the present invention is to provide a method of extendingthe life cycle of a filter in general, and a paper filter in particular,by reducing the tendency of the smaller particles in the fluid beingfiltered to clog the filter body. Another object of the invention is toprovide a novel construction of filter in general, and paper filter inparticular, having an extended life cycle so as to require cleaning orreplacement at less frequent intervals. The invention can also beutilized as a pre-filter, or as a means for improving the effectivenessof cyclone pre-filters or other inertia particle separators.

A method of operating a filter to remove solid particles from a fluid bypassing the fluid through a filter body having an upstream side and adownstream side, characterized in applying to the fluid, solely at theupstream side of the filter body, an AC electrical field of a frequencyand intensity to agglomerate solid particles in the fluid beforereaching the upstream side of the filter body, such as to reduce thenumber of small particles in the fluid tending to clog the filter bodyand thereby to extend the life cycle of the filter body before cleaningor replacement thereof is required.

As will be described more particularly below, the novel concept of thepresent invention is based on the effect of an alternating electricfield on solid particles before entering the passageways between thediscs described in U.S. Pat. Nos. 5,797,978 and 6,391,097, or beforegetting into the fiber thicket zone in paper type filters. This effectwas discovered in a lab test when the operation of a disc filter madeaccording to U.S. Pat. No. 5,797,978 was switched from DC voltage to ACvoltage. The RMS value of the AC voltage was lower by 20% than the DCvoltage. This change resulted with a surprising and unexpected increaseof the measured blocking efficiency, from 99.44% to 99.84%. Theinterpretation of this result is an improvement of 0.4% out of 0.7%,i.e., a dramatic blocking of an additional 57% of the penetratingparticles.

Further investigation of this discovery led to the conclusion that thealternating field induces oscillatory motion of the charged dustparticles and accordingly enhances the kinetics of colliding with eachother. Smaller particles can therefore attach to bigger particles and/orto form agglomerates that can no longer cross the filtrationpassageways. The clogging rate of paper filters depends on the particledistribution. It is known that the smaller particles increase theclogging rate. This led to the concept of using an alternating electricfield as a useful means for increasing the life cycle of paper filters.Subsequent tests in which alternating electric field was generatedaround a paper filter proved that the increase of the filter'srestriction rate was considerably smaller and could accommodate 40% moredust before clogging. Implementation of this method with cyclonepre-filters or other inertia particle separators will evidently improvetheir effectiveness as resulting from the efficiency test of the discfilter described above.

As will also be described more particularly below, the above method isparticularly effective when the filter body is of a fibrous material forremoving solid particles from air or other gas.

In some described preferred embodiments, the AC electrical field isapplied between a plurality of spaced, electrically-conductive grids atthe upstream side of the filter body. Each grid is made with openingswide enough to allow free inflow to the filter body, and the spacingbetween the grids is sufficiently large to prevent voltage breakdowntherebetween.

In another described preferred embodiment, the AC electrical field isapplied between an electrically-conductive housing and one or moreelectrically-conductive grid spaced from the housing, all located at theupstream side of the filter body. Each grid is made with openings wideenough to allow free inflow to the filter body, and the spacing betweeneach two of the electrically-conductive grids and the housing issufficiently large to prevent voltage breakdown therebetween.

A still further embodiment is described below wherein the AC electricalfield is applied between a plurality of coaxially-spacedelectrically-conductive tubes at the upstream side of the filter body.

In the described preferred embodiments, the AC electrical field ispreferably of a frequency of preferably 20-200 Hz, and of an intensityof 1-15 kilovolts.

A filter including a filter body having an upstream side and adownstream side for removing solid particles from a fluid, characterizedin that said filter includes an AC electrical field generator forgenerating an AC electrical field solely on the upstream side of thefilter body of a frequency and intensity to agglomerate solid particlesin the fluid before reaching the upstream side of the filter body, suchas to reduce the number of small particles in the fluid tending to clogthe filter body.

Further features and advantages of the invention will be apparent fromthe description below.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1 is a three-dimensional view, partly broken away, illustrating oneform of filter constructed in accordance with the present invention;

FIG. 2 is a chart comparing the clogging rate of a filter constructedaccording to FIG. 1 with a filter of conventional construction; and

FIGS. 3-6 are fragmentary views illustrating other constructions offilters in accordance with the present invention.

It is to be understood that the foregoing drawings, and the descriptionbelow, are provided primarily for purposes of facilitating understandingthe conceptual aspects of the invention and various possible embodimentsthereof, including what is presently considered to be a preferredembodiment. In the interest of clarity and brevity, no attempt is madeto provide more details than necessary to enable one skilled in the art,using routine skill and design, to understand and practice the describedinvention. It is to be further understood that the embodiments describedare for purposes of example only, and that the invention is capable ofbeing embodied in other forms and applications than described herein.

DESCRIPTION OF PREFERRED EMBODIMENTS

The filter illustrated in FIG. 1, therein generally designated 10,includes a cylindrical filter body 11 of paper, a first cylindrical grid12 of electrically-conductive material enclosing the cylindrical filterbody 11, and a second electrically-conductive grid 13 of cylindricalconfiguration enclosing grid 12 but radially spaced therefrom by aplurality of insulating spaces 14. The two electrically-conductive grids12 and 13 may be of metal braided wire defining a plurality ofrelatively large openings to permit easy passage therethrough of thefluid being filtered. In the example illustrated in FIG. 1, the fluidbeing filtered is air moving from the outer surface of the cylindricalpaper filter 11 towards its inner surface, such that the outer surfaceof filter body 11 constitutes its upstream surface, and the innerconstitutes its downstream surface. Accordingly, the two grids 12 and 13are located in spaced relationship to each other at the upstream surfaceof the paper filter body 11.

Each of the two grids 12, 13, includes a terminal 12 a, 13 a,respectively, connecting the respective grid to a source of AC voltage15. In this example, the illustrated filter is used in a vehicle forfiltering the air inputted into the engine; accordingly, the AC voltagesource 15 is a DC to AC converter powered by the vehicle battery 16controlled by the ON/OFF switch 17.

During the operation of the filter illustrated in FIG. 1, the two metalgrids 12, 13, enclosing the upstream side of filter body 11 and poweredby converter 15, generate an AC electrical field of a frequency andintensity to agglomerate particles in the air reaching the filter body11, such as to reduce the number of smaller particles in the air tendingto clog the filter body. By so reducing the number of smaller particlesin the air tending to clog the filter body, the life cycle of the filterbody is substantially extended before cleaning or replacement isrequired.

The following tests were performed on a filter constructed in accordancewith FIG. 1:

A commercial vehicle filter was used having an initial flow of 200 CFMwith a standard fine test dust fed at a rate of 1 gr/min. The AC voltagewas supplied by a transformer of output 6600V peak-to-peak at 50 Hz. Thegap between the grids 12, 13 was 12 mm. The field intensity accordinglywas over 500V/mm. Prediction of the effect of the electric field on thekinetics of the dust particles is based on the following mathematicalmodel.

The electrostatic charges on dust particles are closely proportional tothe particle size. For example an 0.5 micron particle carries a chargeof 120 e, a 1.0 micron particle carries 250 e and a 1.5 micron particlecarries 420 e (where e=1.6×10⁻¹⁹ coulomb, is one electron charge). Themass of the particle is however closely proportional to the third powerof its size (diameter). The force acting on a charged particle in anelectric field is proportional to its charge Q times the field intensityE, where E=V/d with V being the applied voltage, and d being the gapbetween the electrodes. The acceleration of the particle of mass M isaccordingly a=Q.V/d.M. By substituting the proper values for a 1 micronparticle one can find that for E/d=500V/mm the calculated accelerationis roughly 1500 g. Such a strong acceleration will definitely have aconsiderable impact on the kinetics of the particles. The calculatedfree traveling path at a time interval of 5 milliseconds (¼ of theperiod of the 50 Hz frequency) under such acceleration is roughly 19 cm.Compared with the mean free path of a gas molecules at room temperatureand atmospheric pressure, which is less than 1 micron, the above resultdemonstrates the enormous effect of the alternating electric field onthe kinetics of the dust particles.

The actual test results are presented in FIG. 2, wherein the filtersrestriction is plotted versus the amount of dust loaded in the filter.Curve A demonstrates the test data from a reference bare filter body,and curve B for an identical paper filter having a double grid as shownin FIG. 1. A definite reduction of the clogging rate is observed forcurve B. Restriction was monitored by manometers scaled to mm of water,where each mm equals 0.1 milibars. The restriction of the referencefilter body was increased by 250 mm (water) at 100 grams of dust,whereas it takes 140 grams of dust for the same increase of restrictionin the second grid wrapped filter. This represents an improvement of40%.

It will be appreciated that these test results are provided herein justas an example and that after optimizing design parameters, such as fieldintensity and frequency, the degree of improvement may be higher than40%.

FIG. 3 illustrates a filter, therein generally designated 20, which isof basically the same construction as described above with respect toFIG. 1, but including a third grid of electrically-conductive materialfor generating the AC electrical field at the upstream side of thefilter body. Thus, the filter illustrated in FIG. 3 also includes acylindrical paper filter body 21 enclosed by a first grid 22, a secondgrid 23 spaced therefrom by insulators 24, and a third grid 28 spaced byinsulators 29 from the second grid 23; and an AC power supply includingan converter 25 powered by a battery 26 via ON/OFF switch 27. Thestructure of elements 21-27 in the filter of FIG. 3 thereby correspondsto the structure of elements 11-17, respectively, in the filter of FIG.1.

However, as shown in FIG. 3, terminal 22 a of the inner grid 22, andterminal 28 a of the outer grid 28, are connected together and to oneterminal of the AC voltage source 25, whereas terminal 23 a of themiddle grid 23 is connected to the other terminal of the AC voltagesource 25. Accordingly, grids 22 and 23 define one cylindrical chamberin which a first AC electrical field of one phase is generated; whereasgrids 23 and 28 define a second cylindrical chamber in which a second ACelectric field of opposite phase is generated. The configurationillustrated in FIG. 3 thus creates two successive cylindrical chambersin each of which an AC electrical field is generated to act on theparticles within the air in the same manner as described above withrespect to FIG. 1, such that the particles in the air flow will beaffected by the two electrical fields in series. The describedarrangement thus increases the agglomeration of solid particles reachingthe upstream side of the filter body 21, such as to further reduce thenumber of small particles in the air tending to clog the filter body.

It will be appreciated that additional grids could be added in someapplications, in order to further increase the agglomerating effect onthe dust particles reaching the upstream surface of the filter body.

FIG. 4 illustrates a filter construction similar to that of FIG. 1,except that the AC electrical field applied to the fluid upstream of thefilter body is generated by a metal canister or housing enclosing thepaper filter body, and an electrically-conductive grid between the metalhousing and filter body. Thus, the filter illustrated in FIG. 4, andtherein generally designated 30, includes a cylindrical filter body 31,a metal grid 32 enclosing the outer surface of the filter body, and anouter metal housing 33 enclosing metal grid 32 and spaced therefrom by aplurality of spacers 34. The metal grid 32 includes a terminal 32 aconnected to one side of an AC power source 35, and the metal housing 33is connected via its terminal 33 a to the other side of the powersupply.

As in the previously described embodiments, the outer surface of filterbody 31 is its upstream side, whereas the inner surface is itsdownstream side. FIG. 4 illustrates an inlet pipe 36 coupled to themetal housing 33 for supplying the fluid (air) to be filtered, and theoutlet tube 37 for discharging the filtered air.

The filter illustrated in FIG. 4 is otherwise constructed and operatesin the same manner as described above with respect to FIG. 1.

FIG. 5 illustrates a filter, therein generally designated 40,constructed in accordance with the present invention, wherein the filterbody 41 is of a flat configuration, rather than of a cylindricalconfiguration as in the previously described constructions.

A pair of metal grids 42, 43, spaced from each other by insulatingspacers 44, are located over one side of the flat filter body 41,constituting the upstream side of the filter body. Grids 42 and 43include terminals 42 a, 43 a, connected to an AC power supply 45 forgenerating an AC electrical field to agglomerate solid particles in theair reaching the filter body 41. In all other respects, the flat filterillustrated in FIG. 5 is constructed and operates in the same manner asdescribed above with respect to FIG. 1.

FIG. 6 illustrates a filter wherein the AC electrical field foragglomerating solid particles in the fluid reaching the filter body isgenerated in the inlet tube to the filter body. Thus, the filterillustrated in FIG. 6, therein generally designated 50, includes afilter body 51 within a separate housing 52, and connected at itsupstream side to a metal inlet tube 53 enclosing a plurality of furthermetal tubular electrodes 54, 55 and 56, coaxial with respect to eachother and with respect to the outer metal tube 53. Filter body 51 couldbe a paper filter, a cyclone pre-filter, or any other filter-typedevice.

Each of the metal tubular electrodes 53-56 includes an electricalterminal 53 a-56 a, respectively, connected to an AC power supply 57,with alternate terminals connected to one side of the power supply, andthe remaining alternate terminals connected to the opposite side of thepower supply. Thus, as seen in FIG. 6, terminals 53 a and 55 a areconnected to one side of power supply 57, and terminals 54 a and 56 aare connected to the opposite side of the power supply. Such anarrangement thus produces three coaxial cylindrical chambers at theupstream side of filter 51 within housing 52, in each of which chambersis generated an AC electrical field tending to agglomerate the solidparticles in the air reaching the filter body 51.

It will be appreciated that many other variations and applications ofthe invention may be made. Thus, the invention may be embodied in othertypes of filters, in cyclone pre-filters or other inertia particleseparators and pre-filters, and in other types of filters for removingdust and other solid particles from other fluids, such as other gases.The grid and housings described above as being of metal could be made ofother electrically-conductive materials, such as electrically-conductiveplastics. In addition, the filter body could be of another configurationthan cylindrical or flat, and could be of other constructions ratherthan paper or other fibrous materials. Further, the filter could beequipped with grids or the like for generating a larger number of ACelectrical fields at the upstream side of the filter body in order toagglomerate some of the particles in the fluid reaching the filter body.Many other variations, modifications and applications will be apparent.

What is claimed is:
 1. A method of operating a filter to remove solidparticles from a fluid by passing the fluid through a filter body havingan upstream side and a downstream side, characterized in applying to thefluid, solely at the upstream side of the filter body, an AC electricalfield of a frequency and intensity to agglomerate solid particles in thefluid before reaching the upstream side of the filter body, such as toreduce the number of small particles in the fluid tending to clog thefilter body and thereby to extend the life cycle of the filter bodybefore cleaning or replacement thereof is required.
 2. The methodaccording to claim 1, wherein said filter body is of a fibrous materialfor removing solid particles from air or other gas.
 3. The methodaccording to claim 1, wherein said AC electrical field is appliedbetween a plurality of spaced, electrically-conductive grids all at theupstream side of the filter body, the spacing between said grids beingsufficiently large to prevent voltage breakdown therebetween.
 4. Themethod according to claim 3, wherein said filter body is of cylindricalconfiguration, and said plurality of spaced electrically-conductivegrids are located coaxially around said cylindrical filter body.
 5. Themethod according to claim 3, wherein said filter body is of a flatconfiguration, and said plurality of spaced electrically-conductivegrids are located on one side of said filter body.
 6. The methodaccording to claim 1, wherein said AC electrical field is appliedbetween an electrically-conductive housing and anelectrically-conductive grid spaced from said housing, both located atthe upstream side of the filter body, the spacing between saidelectrically-conductive grid and housing being sufficiently large toprevent voltage breakdown there between.
 7. The method according toclaim 6, wherein; said filter body is of cylindrical configuration; saidelectrically-conductive housing is of cylindrical configurationenclosing said cylindrical filter body; and said electrically-conductivegrid is of cylindrical configuration and is interposed between saidhousing and said filter body.
 8. The method according to claim 1,wherein said AC electrical field is applied between a plurality ofcoaxially-spaced electrically-conductive tubular electrodes at theupstream side of the filter body.
 9. The method according to claim 1,wherein said AC electrical field is of a frequency of 20-200 Hz and anintensity of 1-15 kilovolts.
 10. The method according to claim 1,wherein said filter body is of paper for removing solid particles fromair.
 11. A filter including a filter body having an upstream side and adownstream side for removing solid particles from a fluid, characterizedin that said filter includes an AC electrical field generator forgenerating an AC electrical field solely on the upstream side of thefilter body of a frequency and intensity to agglomerate solid particlesin the fluid before reaching the upstream side of the filter body, suchas to reduce the number of small particles in the fluid tending to clogthe filter body.
 12. The filter according to claim 11, wherein saidfilter body is of a fibrous material for removing solid particles fromair or other gas.
 13. The filter according to claim 11, wherein said ACelectrical fields generator includes a plurality of spaced,electrically-conductive grids all at the upstream side of the filterbody, the spacing between said grids being sufficiently large to preventvoltage breakdown therebetween.
 14. The filter according to claim 13,wherein said filter body is of cylindrical configuration, and saidplurality of spaced electrically-conductive grids are located coaxiallyaround said cylindrical filter body.
 15. The filter according to claim13, wherein said filter body is of a flat configuration, and saidplurality of spaced electrically-conductive grids are located on oneside thereof.
 16. The filter according to claim 11, wherein said ACelectrical field generator includes an electrically-conductive housingand an electrically-conductive grid spaced from said housing bothlocated at the upstream side of the filter body, the spacing betweensaid electrically-conductive grid and housing being sufficiently largeto prevent voltage breakdown therebetween.
 17. The filter according toclaim 16, wherein: said filter body is of cylindrical configuration;said electrically-conductive housing is of cylindrical configurationenclosing said cylindrical filter body; and said electrically-conductivegrid is of cylindrical configuration and is interposed between saidhousing and said filter body.
 18. The filter according to claim 11,wherein said AC electrical field generator includes a plurality ofcoaxially-spaced electrically-conductive tubular electrodes at theupstream side of the filter body.
 19. The filter according to claim 11,wherein said AC electrical field generator generates an electrical fieldof a frequency of 20-200 Hz and an intensity of 1-15 kilovolts.
 20. Apre-filter for use with a filter body removing solid particles from afluid flowing therethrough from an upstream side to a downstream side ofthe filter body, said pre-filter comprising an AC electrical fieldgenerator configured and dimensioned to be applied solely to theupstream side of the filter body for generating an electrical field of afrequency and intensity to agglomerate solid particles in the fluidbefore reaching the upstream side of the filter body, such as to reducethe number of small particles in the fluid reaching the upstream side ofthe filter body and thereby to reduce its clogging rate and to extendits useful life before cleaning or replacement is required.